Library Coq.Sorting.PermutSetoid

Indeed, this file defines a notion of permutation based on
multisets (there exists a permutation between two lists iff every
elements have the same multiplicity in the two lists) which
requires a more complex apparatus (the equipment of the domain
with a decidable equality) than Permutation in Permutation.v.

The relation between the two relations are in lemma
permutation_Permutation.

File Permutation concerns Leibniz equality : it shows in particular
that List.Permutation and permutation are equivalent in this context.